A total of 130 subjects with COPD and chronic bronchitis will be randomized (in a double-blind fashion) to receive either high-dose NAC (1800 mg) or matching placebo twice daily for eight weeks. Respiratory health status, assessed by changes in St. George's Respiratory Questionnaire, will be used to determine the effects of NAC on lung function. It is anticipated that the subjects on high dose NAC will have improved scores.